@Simon As of 1.1, you can increase your health to 500. As William Jackson points out in his answer, you can use Life Fruits, which are only found in hardmode, to increase your max health to 500. However, you have to use Life Crystals to get to 400 max health; there is no other way to increase health, and Life Fruits won't work until you've used 15 Life Crystals.
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KevinFeb 19 at 23:30

After you have 400 health and defeat Wall of Flesh to enter Hardmode, you can consume up to 20 Life Fruit to get to 500 max health. Life Fruit are found in the Underground Jungle and give +5 to max health.

You will need 15 Life Crystals and 20 Life Fruit to get to the maximum of 500 health.